The present invention relates to a method and apparatus for directional radio communication in which signals between a first station and a second station may be transmitted only in certain directions. In particular, but not exclusively, the present invention is applicable to cellular communication networks using an antenna array in an adaptive manner, for example space division multiple access (SDMA).
With currently implemented cellular communication networks, a base transceiver station (BTS) is provided which transmits signals intended for a given mobile station (MS), which may be a mobile telephone, throughout a cell or cell sector served by a base transceiver station. However, space division multiple access (SDMA) systems have now been proposed. In a space division multiple access system, the base transceiver station will not transmit signals intended for a given mobile station throughout the cell or cell sector but will only transmit the signal in the beam direction in which a signal from the mobile station is received so that the same frequency can be used in the same cell at the same time to support two different users. SDMA systems may also permit the base transceiver station to determine the direction from which signals from the mobile station are received.
One problem is that the direction in which a signal is to be transmitted by the base station to the mobile station is determined on the basis of the signals received by the base station from the mobile station. However, in a frequency division duplex mode of operation, the frequencies of signals transmitted from the base station to the base station are quite different from frequencies used for the signals transmitted by the base station to the mobile station. The difference in the frequencies used in the up and down link signals means that the behaviour of the channel in the up link direction may be different from the behaviour of the channel in the down link channel.
In a time division duplex mode, the frequency used in the up and down link channels may be the same. It should be noted that in the time division duplex mode, signals will not be transmitted at the same time in that the up and down link channels. It can be assumed that the characteristics of the up and down link channels are the same provided that the duplexing time is much shorter than the coherence time of the channel. The coherence time of the channel is the interval over which a transmitted symbol will be relatively undisturbed by channel fluctuations. Channel fluctuations may be caused by changes in the radio environment, for example caused by movement of the mobile station. Accordingly, for the assumption that the up and down link channels are the same to be valid, a signal should be sent on the down link channel within a time period from receipt of the up link signal which is less than the coherence time for the channel. If a mobile station is moving, it is less likely than if the mobile station is stationary that the signal to be transmitted in the down link direction will be transmitted within a time period which is legs than the coherence time. Even if the mobile station is not moving the time between the receipt and transmission of a signal may be greater than the coherence time.
The spatial characteristics of an up link or a down link channel, such as its impulse response, will vary greatly in dependence on the radio environment.
In one known system which attempts to deal with this problem is shown in FIG. 1, the radio environment is divided into macro cells A, micro cells B and/or pico cells C. Macro cells A are relatively large whilst micro and pico cells B and C are much smaller. Pico cells C are smaller than micro cells B. Micro and/or pico cells B and C can be contained in macro cells A, as shown in FIG. 1. It is assumed that the radio signal will behave quite differently in macro cells A as compared to micro or pico cells B and C. Thus, this allows different types of strategy to be used in determining the parameters of the signal to be transmitted in the down link direction. However, with this system, the behaviour of the signals in the macro and micro or pico cells can be misleading. For example, the signal of a mobile station which is operating close to a base station in a macro cell A can be received by the base station with a wide angular spread. This type of behaviour is typical of micro or pica cells. Street channelling, which is caused by the presence of buildings, results in a signal being received with a narrow angular spread. If this type of behaviour occurs in a micro or pico cell, the base station will receive the signal with a narrow angular spread. This behaviour is more typical of macro cells. This model therefore does not always reflect the actual radio environment in a macro, micro or pica cell. Rather, the model relies on the expected radio environment given the position of the mobile station relative to the base transceiver station. Accordingly, this model will sometimes result in poor quality of signals.
It is an aim of certain embodiments of the present invention to provide a method and apparatus which avoids or at least mitigates this problem.
According to one aspect of the present invention, there is provided a method of directional radio communication between a first station and a second station, comprising the steps of determining from at least one signal received at the first station from the second station which of a plurality of different radio environment types is present between said first and second stations; and transmitting a signal from the first station to the second station, at least one parameter of said transmitted signal being dependent on the determined radio environment type.
As the radio environment type is determined in accordance with the at least one signal which is received, it reflects the actual radio environment and not the expected radio environment. Thus, the signal transmitted by the first station can better take into account the actual radio environment than with the system described hereinbefore.
Preferably, the radio environment types are determined from the angular spread of the signal received by the first station from the second station. This is a useful parameter of the received signal to use when determining the radio environment type in a directional radio system.
Preferably, a first radio environment type is determined to as be present when the at least one signal received by the first station from the second station has a relatively small angular spread and a second radio environment type is determined to be present when at the least one signal received by the first station from the second station has a relatively large angular spread. In embodiments of the present invention, there may be more than two types of radio environment. However, it is preferred that there only be two as this provides good results but is not overly complex.
Preferably, if the first radio environment is determined to be present, then the signal transmitted by the first station is transmitted with a relatively small beam spread and if the second radio environment is determined to be present then the signal is transmitted with a relatively large beam spread. Thus, the signal which is transmitted takes into account the characteristics of the determined radio environment type.
Preferably, the method further comprises the step of, if it is determined that the first radio environment type is present, determining if the beam spread of the signal to be transmitted by the first station is to be increased. Thus the method ensures that a narrow beam is used whenever it is appropriate. However, if there is any other indication that a wider beam spread should be used to avoid loss of communication or poor quality communication, then the beam spread can be increased.
The determining if the beam spread is to be increased step may comprise considering a parameter indicative of the distance between the first and second stations and increasing the beam spread of the signal to be transmitted to the second station of the distance between the first and second stations is relatively small. The parameter indicative of the distance between the first and second stations may be timing advance information in for example a GSM system. The beam spread of the signal to be transmitted may be increased, when the first radio environment type is present, if the distance between the first and second stations is less than a predetermined threshold.
Alternatively or additionally, the determining if the beam spread is to be increased step comprises considering a parameter indicative of the speed of movement of the second station relative to the first station and increasing the beam spread of the signal to be transmitted to the second station if the relative speed is relatively high. Again, a threshold value can be used so that if the relative speed is greater than a predetermined threshold, then the beam spread of the signal to be transmitted is increased.
In an alternative embodiment, the determining if the beam spread is to be increased step comprises considering a parameter indicative of the relative mobility of the second station relative to the first station and increasing the beam spread of the signal to be transmitted to the second station if the relative mobility is relatively large. Relative mobility takes into account the velocity of the second station relative the first station and the distance between the first and second stations. A threshold value may be used so that the beam spread of the signal to be transmitted is increased if the relative mobility is greater than a predetermined threshold.
Preferably said determining if the beam spread is to be increased step comprises considering the height of an antenna array of the first station above the surroundings and if the antenna array is relatively low with respect to the surroundings the beam spread is increased. A threshold value may of course also be used.
The determining if the beam spread is to be increased step may alternatively or additionally comprise determining the average angular spread for N-previous signals received from the second station by the first station. In, for example, a GSM system, the N-previous signals may be N-data bursts or slots in the case of CDMA systems. Preferably, the signal is transmitted by the first station with a beam spread similar to or the same as the average angular spread which has been determined. There may be a minimum beam spread which is used in the large angular spread environment. The period over which the average is determined preferably takes into account one or more of the following parameters: the distance between the first and second stations, velocity of the second station with respect to the first station, and relative mobility of the second station relative to the first station. If the second station is moving, then the period of time over which the average is taken is preferably decreased.
Preferably, the beam spread of the signal to be transmitted is not reduced if the first radio environment is present, only increased if appropriate.
Preferably, the method further comprises the step of, if it is determined that the second radio environment type is present, determining if the beam spread of the signal to be transmitted by the first station is to be decreased. Typically, if it is determined that the second radio environment type is present, the signal transmitted by the first station will typically have a wide beam spread. With this method, it is determined whether or not that beam spread can be reduced, which allows capacity to be improved.
The step of determining if the beam spread of the signal is to be decreased may comprise determining two extreme directions of arrival for signals from the second station at the first station for the last N-previous signals (for example the last N data bursts) received by the first station from the second station, and transmitting a signal to the second station with an beam spread defined by the two extreme directions of arrival. The directions of arrival may be the dominant directions of arrival.
Alternatively, the step of determining if the beam spread of the signal is to be decreased may comprise determining the variance of the angular spread of the previous N-signals received by the first station from a second station and transmitting to the second station with a beam spread which is related to the calculated variance. The variants of the dominant directions of arrival of N-preceding signals may be determined.
In both of the methods of determining if the beam spread of the signal is to be decreased, the N-previous signals may be N-previous bursts, if the method is being used in a GSM system or slots if the method is used in CDMA systems.
Preferably, if the large angular spread environment is determined to be present, the beam spread is only decreased, if appropriate, and not increased.
Preferably, the beam spread of the received and transmitted signals are defined by one or more beam directions. The width of the or each beam is preferably variable.
It is preferred that the first station is a bane transceiver station in a cellular telecommunication network. The second station may be a mobile station.
According to a second aspect of the present invention, there is provided a first station for directional radio communication with a second station, comprising: means for receiving signals from the second station; means for determining from at least one signal received by the first station from the second station which of a plurality of different radio environment types is, in use, present between said first and second stations; and transmitting means for transmitting a signal to the second station, at least one parameter of the transmitted signal being dependent on the determined radio environment type.
The radio environment types may be determined from the angular spread of the signal received by the receiving means. Preferably, when a signal from the second station is received by said receiving means with a relatively small angular spread, it is determined by said determining means that the radio environment is of a first type and if the signal from the second a station as received by the receiving means with a relatively large angular spread, it is determined that the radio environment is of a second type.
Preferably, the determining means is arranged to determine if the angular spread of the signal transmitted by the transmitting means can be increased if it is determined that the radio environment of the first type and decreased if the radio environment is of the second type.